It is often desirable to sense the location of subsurface objects from outside of the surface of the material in which it is encased (i.e. an object buried underground). For example, sensing the presence of human-scale subsurface objects (both metallic and non-metallic) at relatively near-surface depths (i.e., between zero and 30 meters) can save time, costly explorative excavation, and avoid possible damage to subsurface objects through unguided excavation. Dangers related to digging up objects, such as explosive land mines or gas utility lines, do not have to be contended with or can be reduced if remote sensing from the surface locates the object prior to excavation.
A number of methods have been developed to locate subsurface objects. Subsurface objects, which can be referred to as anomalies, may have various compositions and also include an air pocket or any void or volume uniquely different than the surrounding homogeneous or predictably non-homogeneous material. Metallic objects can be found relatively easily with devices such as metal detectors and through a host of other technologies, such as Ground Penetrating Radar (GPR). It is, however, much more challenging to find non-metallic subsurface objects. The invention described herein is a passive method and apparatus for detecting both metallic and non-metallic subsurface objects, voids and other anomalies using the natural electromagnetic signal emanating from Earth's interior.
The Earth's interior is a highly dynamic structure comprised of multiple layers with a fluid behavior. As the Earth rotates, portions of this fluid move at different velocities and directions. This motion (as well as other factors including lightning, solar wind and flares, etc.) generate low level electromagnetic signals, which then travel outward and pass through the Earth's surface. One example of this phenomena is the well-known core-dynamo effect that creates the quasi-steady state geomagnetic field within the planet. Heating, conduction, and swirling of molten rock can also produce mechanical and electrical signals that travel towards the surface. As these signals travel towards the Earth's surface, they will be affected by the material through which they travel. This effect may show up as variations in signal strength, signal phase, frequency, etc. As the composition of the material varies, so will its effects on the signal passing through it. By monitoring, over an area, the signals emanating from below the Earth's surface, material variations can be detected. This effect can be employed and adapted to locating subsurface objects, voids or other anomalies.
One method of detecting underground structures and other anomalies is audio magneto tellurics (AMT), which monitors AC-signals in the audio frequency range to discover extremely large-scale geological structures. These structures, referred to herein as being of geologic scale, include, by way of example, layers of mineral deposits, rock formations, or other natural resources (such as, for example, coal seams). AMT and other known techniques may not be effective for detecting subsurface objects on smaller scales, at higher resolutions, or at shallower depths.
Another method for detecting underground structures and other anomalies is passive magneto tellurics, which relies on natural, lightening-driven atmospheric noise signals, such as lightening and magnetosphere activities. U.S. Pat. Nos. 4,507,611, 4,825,165 and 5,148,110 to Helms, et al., which are incorporated herein by reference in their entireties, disclose such and other methods for detecting subsurface anomalies. U.S. Pat. No. 6,414,492 to Myers, describes another method for detecting geophysical discontinuities in the Earth by measuring the electrical component of the Earth's electromagnetic field at frequencies below 5 kHz.
These identified methods are capable, to varying degrees, of detecting large, or geologic-scale anomalies at significant sub-surface depths. For example, passive magneto tellurics can detect geological-scale anomalies starting at depths from a few tens of meters to many kilometers, but lacks the resolution to detect small, human-scale objects. Similarly, the passive method disclosed in U.S. Patent No. 5,414,492 can detect geologic-scale anomalies at depths greater than 22.5 meters. The identified methods are not, however, capable of detecting human-scale anomalies or detecting both metallic and non-metallic anomalies at more shallow, near-surface depths (i.e., between zero and 30 meters). For example, none of these methods is sufficiently capable of detecting human-scale anomalies, such as plastic pipes, storage tanks, land mines, or other man-made objects (referred to herein as human-scale objects), buried at near-surface depths. Moreover, the identified methods are capable of generating only relatively low-resolution representations or images of detected subsurface anomalies and have limited capability for determining characteristics of detected subsurface anomalies, such as composition.
Thus, there exists a need in the art for methods and apparatus to passively detect human-scale anomalies, to detect both metallic and non-metallic anomalies, to detect anomalies at near-surface depths, to provide higher resolution representations or images of detected subsurface anomalies, and to determine characteristics of detected subsurface anomalies, such as composition, than what presently is known or available in the art.